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Session 6 - The ISM & Molecular Clouds.
Display session, Monday, June 09
South Main Hall,
The observed properties of the diffuse ionized gas (DIG) in our Galaxy are not easily reconcilable with simple photoionization models. Photoionization models, however, can reproduce the observed properties of H \scriptstyle II regions. This suggests that there are different or additional physical processes at work in the DIG. The He \scriptstyle I \lambda 5876/H\alpha line ratio indicates that a soft ionizing spectrum is needed in the DIG while [N \scriptstyle II] \lambda 6583/H\alpha and [S \scriptstyle II] \lambda 6716/H\alpha line ratios indicate that a harder ionizing spectrum is needed if only photoionization is to explain the properties of the DIG. Minter and Spangler (1997, ApJ, in press) have shown that the thermal heating due to the dissipation of the turbulence is likely to be an important heating mechanism in the DIG while being unimportant in H \scriptstyle II regions.
We have developed a model of the DIG whereby it is ionized by a relatively soft ionizing spectrum (T_eff \le 32,000\, K) and is also heated by an additional thermal mechanism: the dissipation of turbulence. This model predicts the same electron temperature, [N \scriptstyle II] \lambda 6583 ratio, [S \scriptstyle II] \lambda 6716 ratio and He \scriptstyle I \lambda 5876 ratio as observed in the DIG. The model suggests that the observed [O \scriptstyle III] \lambda 5007 emission from the diffuse ISM does not originate from the oxygen in the DIG. Without the turbulent thermal heating term, this model will not reproduce the observed properties of the DIG. The dissipation of turbulence may also be important in other phases of the ISM.
